Gaseous fuel engines are used in a wide variety of applications, in contexts such as power generation, vehicle propulsion, gas compression, pumping, and still others. Operation on gaseous fuel offers a number of advantages, including a generally desirable and controllable emissions profile, particularly with respect to oxides of nitrogen or “NOx.” Gaseous fuel engines are also generally well-suited to stoichiometrically lean operation, and gaseous fuels tend to be readily and economically available and transportable. Most gaseous fuel engines are also spark-ignited, enabling relatively precise control over the timing of ignition of a combustion charge in any given cylinder, which capability can be exploited to various ends.
Gaseous fuel engines are commonly deployed at sites such as oil field and gas field sites, landfills, drilling platforms, and some industrial sites, for example, generally due to the ready availability of gaseous fuels, notably natural gas, landfill gas, biogas, and various others including blends. In more recent years there has been interest in deployment of gaseous fuel engines at certain mine sites where naturally occurring natural gas is commonly found. At a coal mine, for example, a certain amount of natural gas will generally tend to reside within the mine, particularly at shaft mines, and is replenished by natural processes when depleted. While the relative abundance of natural gas in a shaft coal mine will typically be relatively low, it can nevertheless be desirable to vent the mine gas for various reasons. In many modern shaft coal mines, the mine gas is more or less continuously ventilated from the mine by forcing fresh air from the surface through the various shafts and passages of the mine.
The desirability of making use of the otherwise wasted natural gas in mine gas is readily apparent, however, the relative proportion of natural gas in mine gas tends to be lower than what most gaseous fuel engines are designed to operate on. The proportion of combustible gaseous fuel in mine gas may be as low as about 4% and typically no higher than about 12-15% depending on the particular mine and ventilation strategy. For this reason, the energy content of the mine gas tends to be relatively low and a substantial proportion of the total gas throughput of a gaseous fuel engine must be dedicated to conveyance of the mine gas if operation is even possible. For these and other reasons, to enable successful and practicable gaseous fuel engine operation on mine gas, expensive equipment may be required, and it still may not be practicable to operate such an engine at all given fluctuations in the relative proportion of combustible fuel in the mine gas stream.
Various strategies have been proposed before relating to blending gaseous fuels and blends to obtain desired properties, such as energy content. U.S. Pat. No. 9,133,779 to Hughes discloses a strategy for blending raw natural gas and methane gas. While the techniques disclosed in Hughes may be advantageous for certain types of off-spec gas or certain applications, there remains room for further innovation in the gaseous fuels and gaseous fuel engine fields.